1. Field of the Invention
Apparatus and process for mixing chemicals with wood pulp.
2. Review of the Prior Art
The following definitions will be used in this application.
Pulping is the changing of wood chips or other wood particulate matter to fibrous form. Chemical pulping requires cooking of the chips in solution with a chemical, and includes partial removal of the coloring matter such as lignin associated with the wood.
Bleaching is the treatment of cellulosic fibers to remove or alter the coloring matter associated with the fibers to allow the fiber to reflect white light more truly.
The standard symbols for pulping and bleaching sequences are:
S=Sulfite PA1 K=Kraft PA1 So=Soda PA1 C=Chlorine PA1 H=Sodium or calcium hypochlorite PA1 E=Alkali extraction, usually with sodium hydroxide PA1 D=Chlorine dioxide PA1 P=Alkaline peroxide PA1 O=Oxygen PA1 A=Acid pretreatment or part treatment PA1 1. Slurry about 5 hand-squeezed grams of pulp stock in a 600-milliliter beaker and remove all shives. PA1 2. Form a hand sheet in a 12.5-centimeter Buckner funnel, washing with an additional 500 milliliters of water. Remove the filter paper from the pulp. PA1 3. Dry the hand sheet for 5 minutes at 99.degree. to 104.degree. C. PA1 4. Remove the hand sheet and weigh 0.426 grams of it. The operation should be done in a constant time of about 45 seconds to ensure the moisture will be constant, since the dry pulp absorbs more moisture. PA1 5. Slurry the weighed pulp sample in a 1-liter beaker containing 700 milliliters of 25.degree. C. tap water. PA1 6. Add 25 milliliters of 4 N sulphuric acid and then 25 milliliters of 0.1000 N potassium permanganate. Start the timer at the start of the permanganate addition. PA1 7. Stop the reaction after exactly 5 minutes by adding 10 milliliters of the 5% potassium iodide solution. PA1 8. Titrate with 0.1000 N sodium thiosulfate. Add a starch indicator near the end of the titration when the solution becomes straw color. The end point is when the blue color disappears. PA1 A=area swept per metric ton, m.sup.2 /t PA1 r.sub.1 =outer radius of the rotor, m PA1 r.sub.2 =inner radius of the rotor, m PA1 R=revolutions per minute of the rotor PA1 N=number of rotors PA1 t=metric tons (Oven Dry Basis) of pulp passing through the mixer per day.
Consistency is the amount of pulp fiber in a slurry, expressed as a percentage of the total weight of the oven dry fiber and the solvent, usually water. It is sometimes called pulp concentration.
The consistency of the pulp will depend upon the type of dewatering equipment used. The following definitions are based on those found in Rydholm Pulping Processes, Interscience Publishers, 1965, pages 862-863 and TAPPI Monograph No. 27, The Bleaching of Pulp, Rapson editor, The Technical Association of Pulp and Paper Industry, 1963, pages 186-187.
Low consistency is from 0-6%, usually between 3 and 5%. It is a suspension that is pumpable in an ordinary centrifugal pump and is obtainable using deckers and filters without press rolls.
Medium consistency is between 6 and 20%. Fifteen percent is a dividing point within the medium-consistency range. Below 15% the consistency can be obtained by filters. This is the consistency of the pulp mat leaving the vacuum drum filters in the brownstock washing system and the bleaching system. The consistency of a slurry from a washer, either a brownstock washer or a bleaching stage washer, is 9-13%. Above 15%, press rolls are needed for dewatering. Rydholm states that the usual range for medium consistency is 10-18%, while Rapson states it is 9-15%. The slurry is pumpable by special machinery even though it is still a coherent liquid phase at higher temperatures and under some compression.
High consistency is from 20-40%. Rydholm states that the usual range is 25-35% and Rapson states that the range is from 20-35%. These consistencies are obtainable only by presses. The liquid phase is completely absorbed by the fibers, and the pulp can be pumped only very short distances. For practical purposes, it is nonpumpable.
Pulp quantity is expressed in several ways.
Oven-dry pulp is considered to be moisture free or bone dry. Its value is determined by drying the pulp in an oven at a temperature of 100.degree. to 105.degree. C. until it reaches constant weight. It usually is considered to have reached constant weight after 24 hours in the oven.
Air-dry pulp is assumed to have a ten percent moisture content. One air-dry ton of pulp is equal to 0.9 oven-dry tons of pulp.
There are two principal types of measurements to determine the completeness of the pulping or bleaching process, the degree of delignification and the brightness of the pulp. There appears to be no correlation between the two because the delignification factor is a measure of residual lignin within the pulp and the brightness is a measure of reflectivity of the pulp sheet.
There are many methods of measuring the degree of delignification of the pulp but most are variations of the permanganate test.
The normal permanganate test provides a permanganate or K number--the number of cubic centimeters of tenth normal potassium permanganate solution consumed by one gram of oven dry pulp under specified conditions. It is determined by TAPPI Standard Test T-214.
The Kappa number is similar to the permanganate number but is measured under carefully controlled conditions and corrected to be the equivalent of a 50% consumption of the permanganate solution in contact with the specimen. The test gives the degree of delignification of pulps through a wider range of delignification than does the permanganate number. It is determined by TAPPI Standard Test T-236.
PBC is also a permanganate test. The test is as follows:
In running the test, the thiosulfate should first be added as rapidly as possible to prevent the liberation of free iodine. During the final part of the titration the thiosulfate is added a drop at a time until the blue color just disappears. The titration should be completed as rapidly as possible to prevent reversion of the solution from occurring.
The PBC number represents the pounds of chlorine needed to completely bleach one hundred pounds of air dried pulp at 20.degree. C. in a single theoretical bleaching stage and is equal to the number of milliliters of potassium permanganate consumed as determined by subtracting the number of milliliters of thiosulfate consumed from the number of milliliters of potassium permanganate added.
Many variables affect the test, but the most important are the sample weight, the reaction temperature and the reaction time.
There are also a number of methods of measuring pulp brightness. It usually is a measure of reflectivity and its value is expressed as a percent of some scale. A standard method is GE brightness which is expressed as a percentage of a maximum GE brightness as determined by TAPPI Standard Method TPD-103.
Another measure of the brightness or delignification is the opacity of the fiber. Opacity as a percent of a standard is determined by TAPPI Standard Test T 425 OS-75.
Pulp yield may be measured in two ways. The first is the amount, by weight, of carbohydrates and lignin returned per unit of wood. Screened yield is closely related and proportional to this chemical return. A high screened yield means the chemical return is high and a low screened yield means the chemical return is low. The second measurement of yield is fiber yield, by weight, per unit of wood. Rejects or screenings are related to and inversely proportional to the fiber yield. A high reject level means there is a low fiber return and a low reject level means there is a high fiber return. The total yield is the sum of these two yields. The ideal situation would be one in which there is a high chemical return and a high fiber return indicated by a high screened yield and low screenings.
There are a great number of devices for mixing wood pulp with chemicals. The following are exemplary.
The process disclosed in the International Paper patents to Roymoulik et al, U.S. Pat. No. 3,832,276, which issued Aug. 27, 1974, and to Phillips, U.S. Pat. No. 3,951,733, which issued Apr. 20, 1976 requires a pulp at a consistency of less than 10 percent, preferably about 2 to 6 percent and most desirably between 3 and 4 percent. The pulp is mixed with oxygen in a high shear mixing device and the slurry is introduced into a vessel. The slurry rises upward through the vessel. There is no substantial agitation of the fibers as they travel upward and the pressure on the pulp is gradually reduced. The maximum pressure difference is between 1 and 10 atmospheres. This is preferably done in a bleach tower having a height of between 40 and 300 feet.
"Generally speaking from about 5 to 120 minutes is sufficient. For the higher initial pressure provided by the higher tower the time can be reduced to a period of from about 1 minute to 60 minutes. With a 40 foot tower providing a pressure differential of roughly about 1 atmosphere about 30 to 60 minutes, preferably about 40 minutes is satisfactory."
The oxygenated pulp does not go directly to the tank. Between the mixer and the tank are a heat exchanger 5, a vent 7, and, optionally, a prepressurizing chamber 6.
The Rauma-Repola system is described in the Federal Republic of Germany patent disclosure No. 24 41 579, Mar. 13, 1975 and in Yrjala et al, New Aspects in Oxygen Bleaching, dated Apr. 18, 1974. The system uses the Vortex mixer shown in FIGS. 2 and 3 of the patent. It is possible, by using either a number of passes through a single mixer or several mixers in series, to bleach the pulp in from 5 to 15 minutes. The consistency is 3%.
Yrjala, et al. "A new reactor for pulp bleaching" Kemian Teollisuus 29, No. 12: 861-869 (1972) describes a chlorine reactor.
Richter U.S. Pat. No. 4,093,506 describes a mixer for mixing bleaching fluids such as chlorine or chlorine dioxide with pulp. Rapidly rotating rotor blades essentially fluidize the pulp and the treatment gas is then added to it. The Kamyr reactor is also described in an article, "Pilot and Commercial Results of Medium Consistency Chlorination," given at the Bleaching Seminar on Chlorination and Caustic Extraction, Nov. 10, 1977 in Washington, D.C.
The TAPPI monograph "The Bleaching of Pulp" describes and shows on pp. 325 and 332, respectively, single-shaft and double-shaft steam mixers. A steam mixer has a swept area of around 6500 square meters per metric ton of oven-dry pulp.
Reinhall U.S. Pat. No. 4,082,233 discloses a refiner having means for removing excess gas before the stock enters the refiner.